Photonics SpectraFeb 2017
A groundbreaking new optical device has been developed to correct images of the sun previously distorted by multiple layers of atmospheric turbulence.

Recent images taken from Big Bear Solar Observatory of a massive section of the Sun’s surface — about 23,000 square miles — showcase the advances in real-time clarity over vast distances presented. Courtesy of Big Bear Solar Observatory.
The device developed at the New Jersey Institute of Technology’s (NJIT) Big Bear Solar Observatory (BBSO) is providing scientists with the most precisely detailed, real-time pictures to date of solar activity occurring across vast stretches of the star's surface.

Philip Goode, research professor of physics at NJIT and the leader of an international team of researchers funded by the National Science Foundation (NSF) to develop this next-generation optical system, said the observatory's 1.6-meter New Solar Telescope can now produce simultaneous images at approximately the same time across large structures.

"To understand the fundamental dynamics of the sun, such as the origin of solar storms, we need to collect data from as wide a field of view as possible," said Goode. "Only by seeing the comprehensive array of eruptions all at once will we be able to accurately measure the size, strength and sequencing of these magnetic events and also analyze the forces that propel the star's magnetic fields to twist around each other until they explode, spewing massive amounts of radiation and particles that, when directed earthward, can cause disruptive space weather."

The multi-conjugate adaptive optics (MCAO) device sits downstream of the aperture of the BBSO telescope, currently the world's highest-resolution solar telescope. The system is composed of three mirrors that change shape to correct the path of the incoming light waves, guided by a computer attached to ultra-fast cameras that take more than 2,000 frames per second to measure aberrations in the wave path. The system is called multi-conjugate because each of the three mirrors captures light from a different altitude — near the ground and at about three and six miles high — and the three corrected images together produce a distortion-free picture that eliminates the effects of turbulence up to about seven miles.

The MCAO system has tripled the size of the corrected field of view now available with the current technology, known as adaptive optics, which employs a single shape-shifting, or deformable, mirror to correct images.

"The gain of using three deformable mirrors instead of one is easily visible. The images are crisp in a much larger area," said Dirk Schmidt, a post-doctoral researcher at the National Solar Observatory (NSO), a project scientist for the international MCAO team.

Turbulent airflows at different layers of the Earth's atmosphere, from the ground up to the jet stream, change the path of the sun's light faster than the human eye can compensate, blurring the images captured by conventional telescopes.

The MCAO team, which includes researchers from NJIT, NSO and the Kiepenheuer Institute for Solar Physics in Germany, has been working together for more than a decade on the next generation of adaptive optics to correct these distortions.

The researchers succeeded in significantly widening the field of view after several years of alternating laboratory experimentation — with an artificial light source functioning as the Sun that emitted light waves purposefully distorted by the heat emanating from hot plates — with "on-sky" tests performed in real time in the BBSO's optical path.

"Finally, late last July, we saw what we had long sought — a continuous stream of sharp, wide-field corrected, but essentially identical images," said Goode. "We then repeated the test several times by looking at various places on the Sun to prove we had succeeded. The final trick was narrowing the field to get a deeper-focused correction with each mirror, much like you would adjust a camera to have the near and far field in focus."

The scientific gains are expected to be multi-level. A clearer, more comprehensive view of solar activity should provide additional clues to researchers seeking to explain mysterious dynamics, such as the means by which explosions on the sun produce magnetic explosions and radiation and accelerate particles to nearly the speed of light within seconds. Andrew Gerrard, director of NJIT's Center for Solar-Terrestrial Research, which operates the BBSO and several other solar instruments around the world and in space, noted that the more scientists understand physical processes taking place more than 90 million miles away, the better policymakers will be able to predict and prepare for solar storms with the ferocity to disrupt communications satellites, knock out GPS systems, shut down air travel and quench lights, computers and telephones in millions of homes and businesses.

The MCAO project also serves as a critical test of optical instruments that will be required by future solar telescopes.

The scientific observation of celestial radiation that has reached the vicinity of Earth, and the interpretation of these observations to determine the characteristics of the extraterrestrial bodies and phenomena that have emitted the radiation.